The present invention concerns an efficient implementation of an analog-to-digital converter, and particularly the fact how a signal imparted with high offset may be digitized efficiently and with high accuracy by analog-to-digital conversion.
In a multiplicity of practical applications, it is essential to determine and digitize a small signal proportion imparted with high, approximately constant offset. This entails a series of problems. For example, if such a signal is digitized with a conventional sigma-delta converter (ΣΔconverter), a pulse sequence of very high density would result. Since the offset itself also has to be measured in addition to the signal proportion of interest located on the offset, and the accuracy needed is determined by the signal proportion located on the offset, the accuracy of the converter would have to be very high. This is due to the fact that the offset signal including no information would have to be digitized with the same accuracy as the signal part located on the offset. This would either necessitate a multi-bit analog-to-digital (AD) conversion or very high oversampling. There are technological boundaries for both alternatives, however. On the one hand, multi-bit converters, such as Flash ADCs, which may digitize the signal in a single conversion step, are extremely expensive, since they need a comparator for each resolution stage, and further are allowed to have only few errors between the stages. This leads to the fact that energy consumption increases heavily with growing resolution, on the one hand, and that a lot of chip area is needed to implement such a converter, on the other hand.
This leads to problems in integrated designs, particularly in applications where the digitization takes place on the chip of a sensor. The increase of the sample frequency, which basically would enable using a converter with lower resolution, is also faced with technological boundaries. Particularly in the case of the increase of the clock frequency, the current consumption rises with the number of switching processes, so that heat development also increases to the same extent. In applications in which the converter is attached in direct geometrical proximity to a sensor element, this may lead to a no-longer-tolerable corruption of the measurements results due to the additional heat input by the additionally introduced heat of the ADC conversion through heating of the converter.
In the following, the readout of a bolometer is to serve as an example for the readout of a sensor. A bolometer serves for measuring heat by absorbing electromagnetic radiation in the bolometer, whereby the temperature of the bolometer rises. By using a temperature-dependent device, the change in the temperature is converted into an electrical signal. Here, for example, resistive elements well insulated thermally from the environment are used in a vacuum, the electrical resistance thereof changing with the heating. The temperature changes through the incident heat radiation are very small, however. Temperature differences of less than 1 mK have to be resolved for the measurement. In the case of the resistance measurement, for example, a constant voltage may be applied to the resistive element, so that the thermally induced resistance change causes a minimally changing current flow to be measured. An offset current large as compared with the thermally induced current changes will already flow due to the intrinsic resistance of the resistive element, however, when applying a voltage. The electrical signal thus is superimposed by a very high offset.
The current often is measured by a bolometer in the above-described configuration with a sigma-delta converter. The offset here is subtracted in different ways. For example, so-called blind bolometers, i.e. bolometers shielded from the incident electromagnetic radiation and well connected thermally to the substrate through which the offset-forming current flows, often are used. The current of the blind bolometer may be subtracted from the current of the sensitive bolometer prior to the analog-to-digital conversion (AD). Typically, the bolometer resistances of the individual bolometers of a bolometer sensor array deviate strongly from each other for manufacturing reasons, however, so that the currents flowing through the bolometer have to be equalized prior to measurement. To this end, in the common implementations, the voltages across the active and blind bolometers are adjusted via DA-converters. For this purpose, some readout circuits use transistors located in the pixels of the bolometer array and the gate voltages of which are capable of being adjusted with a DA-converter. In such a solution, the transistor used for equalizing is not within a locked loop, however, so that deviations may occur easily. The threshold voltage of a transistor is temperature-dependent. Thereby, drifting of the voltage may occur. In the above-described method, an AD converter is used for generating the actual measuring signal. This is problematic, among other things, since the voltage corresponding to the LSB is not identical when using two different converters, so that the determination of the measuring quantity takes place with a precision different from that of the offset compensation.
For bolometer arrays, or generally for sensor arrays, it holds true that the pixels of the sensor array typically are read out sequentially. For example, if an analog-to-digital converter sensor column or sensor row is used, the ADC converter has to be constructed in a very simple way. In particular, multi-stage or multi-bit ADCs are to be avoided, since they need too much chip area, on the one hand, and entail significant heat input in the sensor, on the other hand. If the principle of the sigma-delta conversion is used, the ADC used within the sigma-delta circuit thus should be constructed in a very simple way, i.e. ideally have a resolution of only one bit. When reading out bolometers, in particular, there arises the problem of additional heating through the readout electronics used on the bolometer chip. The electrically introduced power inevitably leads to the fact that the chip itself heats up because of the readout circuit. The generally undesirable property is aggravated further by the fact that the amplifiers or ADCs are arranged on one side of the chip, since the one-sided heat input leads to heat gradients on the chip.
Although 2.5 bits of resolution could be gained when doubling the oversampling ratio of a sigma-delta converter with a second-order modulator, the increase of the oversampling ratio is especially critical, for example in the bolometer readout, since the loss heat generated by the readout circuit increases heavily thereby. This should be compensated for by a simplification of the remaining devices being part the circuit, in order to keep the loss power introduced by the readout circuit on an acceptable level altogether.